Ross MacFarlane offers a set of set of free wargames rules for the late 19th/ early 20th centuries called Sending MacDuff to the Front.
Miniatures Rules
Calvinball Rules
Here are a set of rules for playing Calvinball, from the legendary comic strip Calvin and Hobbes. The rules come with figure cut-outs.
HELLION Universal Tabletop Warfare Simulator
John Bear Ross, digital sculptor extraordinaire, offers HELLION, a set of universal miniatures rules.
Gettysburg At The Gallop
From Baccus Miniatures comes Gettysburg At The Gallop, a set of free wargames rules for the American Civil War in 6mm.
Command Combat Civil War: Bull Run
Command Combat Civil War: Bull Run is both a rules set and a scenario for playing the American Civil War.